The oil and gas exploration industry employ geophysical tools and techniques to identify a subterranean structure having potential hydrocarbon deposits. Commonly referred to as seismic exploration, these techniques and tools generate an image of subsurface structures by recording energy in the form of vibrations reflected or refracted from geologic formations. In seismic exploration, for example, seismic waves generated by a source and imparted into the ground reflect off rocks in the subsurface. Boundaries between different rocks often reflect seismic waves, and information relating to these waves is collected and processed to generate a representation or images of the subsurface.
When seismic waves generated by the source reach a bedding plane separating rocks of different acoustic density, then a portion of the waves reflects back to the surface, causing the ground surface to rise or fall depending on whether the expansion or compression phase of the wave is being recorded. The remaining portion of the waves is refracted and diffracted. A two-dimensional image, which is called a seismic line, is essentially a cross-sectional view of the earth oriented parallel to the line of geophones. The information may also be collected as an intersecting grid of seismic lines referred to as a 3-D seismic volume.
Any number of exploration systems can gather the desired information for processing. Dynamite explosions, vibrator trucks, air guns or the like can create the seismic waves. Sensors such as velocity geophones, accelerometers, and/or hydrophones can be laid out in lines, or towed in the case of hydrophones, to measure how long it takes the waves to leave the seismic source, reflect off a rock boundary, and return to the sensors used.
An example seismic system 10 in FIG. 1 can generate geophysical information to image earth subsurface structures. The system 10 has a central controller/recorder 90 in communication with a seismic acquisition array 12 known as a spread. The array 12 has spaced sensor stations 20, which can each have one or more sensors 22. The sensors 22 measure geophysical information and can include 3-component sensors for obtaining 3-dimensional energy known as 3D seismic. The sensors 22 can include accelerometers, velocity geophones, microphones, or the like, and the array 12 can be deployed on land or a seabed location.
A seismic source 30 imparts acoustic energy into the ground, and the sensors 22 receive energy after reflection and refraction at boundaries in subsurface structures. The array 12 then communicates sensor data with the central controller or recorder 90 using wireless technology or other communication technique.
To impart the acoustic energy, the seismic source 30 can be a vibrator, such as shown in FIG. 2, although other types of sources can be used. The vibrator 30 transmits force to the ground using a baseplate 70 and a reaction mass 50. As is typical for land seismic, the vibrator 30 is mounted on a carrier vehicle (not shown) that uses bars 32/34 to lower the vibrator 30 to the ground. With the vibrator 30 lowered, the weight of the vehicle holds the baseplate 70 engaged with the ground so seismic source signals can be transmitted into the earth.
The reaction mass 50 positions directly above baseplate 70 and stilts 52 extend from the baseplate 70 and through the mass 50 to stabilize it. Internally, the reaction mass 50 has a cylinder 56 formed therein. A vertically extending piston 60 extends through this cylinder 56, and a head 62 on the piston 60 divides the cylinder 56 into chambers. The ends of the piston 60 connect to cross-pieces 54U-L that connect to the stilts 52.
Feet 36 with isolators 40 isolate the baseplate 70 from the bars 34, and tension members 42 interconnect between the feet 36 and the baseplate 70. The tension members 42 hold the baseplate 70 when the vibrator 30 is raised and lowered to the ground. Finally, shock absorbers 44 are also mounted between the bottom of the feet 36 and the baseplate 70 to isolate vibrations therebetween.
During operation, a controller 80 receives signals from a first sensor 85 that measures acceleration of the baseplate 70 and receives signals from a second sensor 87 that measure acceleration of the reaction mass 50. Based on feedback from these sensors 85/87 and a desired sweep signal for operating the vibrator 30, the controller 80 generates a pilot signal to control a servo valve assembly 82. Driven by the drive signal, the servo valve assembly 82 alternatingly routes hydraulic fluid between a hydraulic fluid supply 84 and the piston 60. The reaction mass 50 reciprocally vibrates on the piston 60. In turn, the force generated by the vibrating mass 50 transfers to the baseplate 70 via the stilts 52 and the piston 60 so that the baseplate 70 vibrates at a desired amplitude and frequency or sweep to generate a seismic source signal into the ground.
As the moving reaction mass 50 acts upon the baseplate 70 to impart a seismic source signal into the earth, the signal travels through the ground, reflects at discontinuities and formations, and then travels toward the earth's surface. At the surface, the array 12 of FIG. 1 having the geophone receivers or other sensors 22 coupled to the ground detect the reflected signals, and the recorder 90 of FIG. 1 records the seismic data 92 received from the geophone receivers 22.
At some point, a data processing system 98 receives the seismic data 92 from the seismic recorder 90. (The seismic data 92 can also include recorded data from the seismic vibrator 30 if information such as pilot signal, acceleration data, and weighted sum ground force are stored separately.) The data processing system 98 can use a correlation processor to correlate the computed ground force supplied by the vibrator 30 to the seismic data 92 received by the geophone receivers 22. Ultimately, the correlated information can be used to create an image or representation of the earth's subsurface structures.
When operating such a prior art vibrator 30, operators experience problems in accurately determining the ground force that the vibrator 30 is applying to the ground and in accurately correlating the vibrator's operation with the generated source signal. Ideally, operators would like to know the actual ground force applied by the baseplate 70 to the ground when imparting the seismic energy. As shown in FIG. 2, a local sensor 85 (e.g., accelerometer or geophone) is typically positioned on the upper cross piece 54U of the vibrator 50, which positions above the reaction mass 50.
In operation, the controller 80 shown in FIG. 2 measures the signal imparted into the ground using the local sensor 85 located on the upper cross-piece 54U and using the sensor 87 located on the reaction mass 50. When the data processing system 98 of FIG. 1 receives the seismic data 92 making up the seismic spread, it also receives the acceleration signals from these sensors 85/87 on the source 30. The system 98's correlation processor then uses various algorithms to distinguish wave signal data from distortions and other spurious signals.
A problem with this method is that original source signal distortion may vary and make correlation difficult. When calculated ground force signals at the vibrator 30 are cross-correlated with far-field signals measured in the field, the results may be corrupted by unrealistic assumptions used in modeling the system 10. In particular, the vibrator 30 works on the surface of the ground, which can vary dramatically from location to location due to the presence of sand, rock, vegetation, etc. Thus, the baseplate 70 is often not evenly supported when deployed against the ground at a given location. In addition, the baseplate 70 will flex and directly affect the control system during operation. As a result, the radiated energy produced can vary from location to location depending on where the vibrator 30 is deployed. Therefore, the vibrator's source signature is not the same (or nearly the same) from location to location and is not characteristically repeatable, which is desirable when performing seismic analysis. Thus, a more accurate knowledge of the source signal imparted into the ground by the source 30 can make the correlation easier at the data processing stage.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.